1. Field of the Invention
This invention relates to a constant pressure printing mechanism or a printing pressure buffer mechanism for use in a hand-operated label printing machine for regulating the printing pressure that is applied by the types of the printing head, so as to attain clear and precise printing of labels, without different darkness printed characters and without double printing.
2. Description of the Prior Art
Conventional hand-operated label printing machines include a label printing head which is attached to a hand operated printing lever and which is opposed to the platen. The hand or printing lever is pivotally secured to the machine frame. The printing head is driven by turning of the printing lever with respect to the main body of the label printing machine until the type faces disposed in the lower part of the printing head are brought into contact with a label which is carried on a platen, thereby printing the label.
In such a label printing machine, the printing pressure applied by the printing head to the platen and the label thereon is directly influenced by the intensity of the squeezing force applied by the user to the hand or printing lever. When the hand lever is squeezed weakly, the printing pressure is weak, and the imprint on the label is obscure. When the hand lever is squeezed too strongly, the printing pressure is very large. If the type faces of the printing head are made of a rigid material, such as a metal, minute vibration of the printing head occurs in the direction perpendicular to the surface of the platen, and double printing occurs. If the type faces of the printing head are made of an elastic material, such as rubber, the types are flattened during printing under pressure, so that the imprint becomes obscure due to the spreading of the ink.
Further, when the hand lever is squeezed too strongly, the labels and the platen are often damaged by the type faces which are made of rigid material such as a metal and the labels are often stained. Still further, the types and the platen must be replaced with new ones after only relatively short periods of use, which is disadvantageous.
Even when labels are somewhat indistinctly printed with a conventional label printing machine, they are still accepted because customers can read them when they purchase the labeled commodities and cashiers can also read them when they totalize prices. In recent years, the figures, symbols (bar codes, OCR characters, etc.), and the like that are printed on labels have come to be read by computerized optical readers in place of cashiers in order to put what is called a POS (point of sales) system into practice. In this system, information concerning stocks, sales, kinds of goods, etc. are memorized and processed by electronic computers. Therefore, it has become necessary that the labels to be read always be printed with clear and precise characters and symbols.
Accordingly, in order to exert the proper printing pressure on a label and to perform clear printing, the hand lever must be squeezed by a suitable force. It is, however, quite difficult to control or predict the force of squeezing of the hand lever by all operators.